1. Field of the Invention
The present invention relates to a color printer for recording a color image on photosensitive material, and more particularly to a color printer using a color spatial light modulator having small size mirrors disposed in line or matrix, of each of which the light reflection direction is variable.
2. Description of the Related Art
A color photographic printer for projecting an image of a negative film onto photosensitive material such as color paper is most common as a color printer for recording a color image (including characters and line drawing) on color paper. Recently, a color printer is used in practice which drives a display unit by image data to record a color image on color paper. Such a color printer includes a CRT type, a laser type, a liquid crystal type, and the like.
A CRT type color printer includes a line type and a frame type. The line type uses a flat color CRT drawing red, green, and blue three rasters to form a color image through three-color line-sequential exposure. The frame type uses a fine precision monochrome CRT and a filter turret in which a monochrome image of a three-color image is sequentially formed, and corresponding color filters are set in front of the monochrome CRT to convert the monochrome images into red, green, and blue images and sequentially record them. The CRT type therefore requires a large size CRT and a complicated CRT drive circuit.
A laser color printer uses lasers for generating three-color laser beams and spatial light modulators for modulating the intensity of each laser beam with image data of each color. Intensity modulated laser beams of three colors are synthesized, and the synthesized laser beam is scanned by a polygon mirror or the like in the width direction of color paper to print a color image line-sequentially. With the laser color printer, the intensity of a laser beam is required to be modulated serially in unit of dot so that modulation of one line takes a relatively long time. Furthermore, since an exposure time for one pixel is short, a reciprocity law failure of color paper may occur being unable to reproduce an original image with high fidelity. Still further, a laser color printer cannot perform frame exposure.
A liquid crystal color printer uses as a spatial light modulator a liquid crystal unit having a number of micro light bulbs disposed in line. This liquid crystal color filter has a low transmittance so that a bright light source is necessary, and since it has a low aperture efficiency (vignetting factor), the image quality is degraded. Furthermore, since the performance of a liquid crystal unit changes with time and environments, the tonal level is difficult to be correctly controlled.
A mirror type spatial light modulator is known which has a number of small size mirrors (hereinafter called micromirrors) and the tilt angle of each micromirror is changed to control deflection of spot light. Mirror type spatial light modulators include digital micromirror devices (DMD) which tilt each micromirror by electrostatic force, piezoelectric type drive micromirror devices (AMA) which tilt each micromirror by a fine piezoelectric element, and the like.
For example, a digital micromirror device has a static RAM (SRAM) on each memory cell of which a micromirror capable of swinging is formed by semiconductor integration techniques. Mirror drive data of one bit, when written to each memory cell, tilts the micromirror in a positive direction or in a negative direction to change the light reflection direction. The principle and applications of such a digital micromirror device are described in a monthly magazine "O plus E", October, 1994, pp. 90-94.
The micromirror device has advantages of a low light attenuation coefficient and a large aperture efficiency (vignetting factor). It is therefore advantageous if the micromirror device is utilized in a color printer. In printing a color image by using the monochrome spatial light modulator, a filter turret is used, which is rotated to sequentially enter red, green, and blue sectors into an optical path to perform three-color line-sequential exposure or three-color frame-sequential exposure. This filter turret complicates the structure of a color printer and hinders high speed printing.